The present invention pertains to processing of radio communication signals to offset the effects of multipath fading, time dispersion and channel interference. The invention more specifically pertains to performing the above identified signal processing using signals derived from a variety of different antenna designs, or using signals separated into in-phase and quadrature components.
In a practical radio communication system, there commonly co-exists a number of phenomena which degrade the quality of transmitted signals. Among the most troublesome phenomena are flat fading, time dispersion, and channel interference.
Flat fading and time dispersion are multipath effects caused by the interference of a main transmitted signal and its reflections or echos (which may be caused by the surrounding terrain). When the path lengths are relatively small, the multiple signals will arrive at the receiver at almost the same time. The signals add either constructively or destructively, giving rise to flat fading having a Rayleigh distribution. When the path lengths are relatively large, the transmission medium is considered time dispersive, and the added signals can be viewed as echoes of the transmitted signal. The echoes are delayed with respect to the main ray. Time dispersion gives rise to intersymbol interference (ISI).
Channel interference arises due to the presence of sources which are not orthogonal to the desired signal. Non-orthogonal signals, or interference, often come from radios operating on the same frequency (i.e. co-channel interference) or from radios operating on neighboring frequency bands (i.e. adjacent-channel interference). Non-orthogonal signal sources are referred to as interferers.
Fading can be mitigated by having multiple receive antennas and employing some form of diversity combining, such as selective combining, equal gain combining, or maximal ratio combining. Diversity takes advantage of the fact that the fading on the different antennas is not the same, so that when one antenna has a faded signal, chances are the other antenna does not. Note Mobile Communications Design Fundamentals by William C. Y. Lee, Howard W. Sams & Co., Ind., U.S.A. In section 3.5.1 of this book, several examples are given describing how signals from two receiver amplifiers with separate antennas can be combined to counteract fading.
Time dispersion may be advantageously corrected by using an equalizer. In the case of digital signal modulation, a maximum likelihood sequence estimation (MLSE) equalizer such as described in Digital Communications, 2nd Ed., by John G. Proakis, Mc-Graw Hill Book Company, New York, N.Y., U.S.A., 1989 may be used. In section 6.7 of this book, various methods are described for detecting signals corrupted by time dispersion, or inter-symbol interference (ISI), using MLSE equalization.
The impact of channel interference may be reduced by employing array processing techniques with multiple antennas. For example, adaptive beamforming can be used to "steer" a null in the antenna pattern in the direction of an interferer.
More recently, methods have been proposed that partially solve the problems of multipath fading and interference. In U.S. Pat. No. 5,191,598 to Backstrom, et al., for example, the problem of accurately detecting signals in the presence of flat fading and time dispersion is overcome by using a Viterbi-algorithm having a transmission function estimated for each antenna. By reference thereto, U.S. Pat. No. 5,191,598 is incorporated herein in its entirety. Another method of accurately detecting signals in the presence of flat fading and interference was presented in the IEEE Transactions on Vehicular Technology, Vol. 42, No. 4, Nov. 1993, J. H. Winters: "Signal Acquisition and Tracking with Adaptive Arrays in the Digital Mobile Radio System IS-54 with Flat Fading".
Although the above described techniques greatly improve signal quality, there remains room for improvement. Including separate processing modules to separately address fading, time dispersion and interference adds to the cost and complexity to the device.
Furthermore, algorithms (such as Viterbi's) can not cost-effectively handle a large number of diversity branches. It would be desirable to provide some way of culling out a smaller number of diversity branches prior to performing interference rejection and diversity combining to reduce the computational complexity and hardware requirements of the receiver.
Finally, the above-described techniques fail to provide a receiver design which could be easily adapted to a variety of antenna configurations, such as phased array antennas or antennas producing signals having different polarizations. It would be desirable to provide a technique for reducing the deleterious effects of signal transmission which would accommodate a wide variety of antenna configurations. There is further need to reduce deleterious effects of signal transmission in the situation where the in-phase and quadrature components of the interference are correlated.